The goal of air-to-air passive ranging is to determine the range (distance) from one aircraft, called “ownship”, to another, called “target”, by means of electromagnetic energy emanating from target. For this purpose, ownship is equipped with directional receivers operable to determine the angle of arrival of the incoming energy at various times during a data collection interval. In most applications, the energy detected will be in the radio frequency (RF) or infrared (IR) portions of the spectrum. By combining the directional measurements with measurements of ownship's position and heading, the range between the two aircraft may be determined. The dominant problem in air-to-air passive ranging is ill-conditioning. Ill-conditioning refers to small errors in the directional measurements causing much larger errors in the range estimate. The sources of ill-conditioning include the limited baseline over which data is measured and the need to infer target motion from the data.
Range is estimated from bearing measurements at different locations using some variation of a “triangulation principle”. A “baseline” is the distance between two points at which the bearing measurements, called sightings, are taken. Other factors being equal, sightings projected from a long baseline intersect at a greater angle than those taken from a short baseline. Since sightings projected from a short baseline are nearly parallel, a small error in the measured angle results in a large error in determination of range. Hence, range measurements determined using a long baseline are much more impervious to noise contained in the bearing measurements than are those determined over a short baseline. However, the desire to determine range as quickly as possible restricts the baseline that may be developed during data collection. Furthermore, flight path geometry also dictates the maximum baseline which may be used; for example, if two aircraft are flying towards one another, no baseline will be generated regardless of time.
In air-to-ground ranging, the target can be presumed as being fixed in space. Thus, it can be described by a static model consisting of its coordinates in 3-dimensional space. However, this is not true of the air-to-air problem, where target motion must be considered. The additional degrees-of-freedom implied by the dynamic model must be resolved from the data. This aggravates ill-conditioning because it diminishes the redundancy by which information may be distinguished from noise in the data. Unresolved noise in the data produces ranging errors.
As established by the foregoing, a need has arisen to provide a method and apparatus for measuring the range from one aircraft to another, capable of supplying ranging information quickly, while minimizing the effect of small errors in data measurement on the ranging determination.